Kováts retention indexes of monounsaturated C12, C14, and C16 alcohols, acetates and aldehydes commonly found in lepidopteran pheromone blends.

摘要

To date, pheromone blends have been identified for more than 1,000 lepidopteran species1. A substantial fraction of the components that comprise these blends are monounsaturated straight-chain alcohols, aldehydes and acetates with chain lengths of 12, 14, or 16 carbons. These compounds are usually found in amounts that vary from picograms to micrograms per individual, depending on species. Consequently, they are rarely isolated in quantities sufficient to carry out the full range of spectroscopic tests (1H and 13C NMR, infrared, and mass spectrometry) normally used to characterise organic compounds. Instead, identifications usually have relied on mass spectrometry (MS), and particularly MS coupled with gas chromato-graphy, which provides both the separation of most compounds in a crude extract of the pheromone glands, and yields a full scan mass spectrum on any compound present in amounts of a few nanograms or more. Microchemical tests also may be used to determine the presence or absence of specific functional groups2. However, the amount of information that can be obtained from these methods is somewhat limited, and conducting microchemical tests on nanogram to picogram quantities of compounds without losing them altogether can be technically challenging. The problem is exacerbated by the fact that whereas mass spectrometry can frequently be used to determine the gross structure of a monounsaturated alcohol, aldehyde, or acetate pheromone component, in general the mass spectrum provides insufficient information to determine either the position or the geometry of the double bond reliably; for example, the mass spectra of most monounsaturated acetates of a given chain length are quite similar. Furthermore, because different types of mass spectrometers (e.g., quadrupole, magnetic sector, or ion trap) produce slightly different spectra, matchups with literature spectra cannot be relied upon for determinations of double bond positions or geometries. Because of these problems, GC retention time data has been used extensively in the identification of lepidopteran pheromones, with matching retention times between an unknown compound and a standard of known structure providing strong support for a tentative identification. However, a retention time match is not conclusive, because isomers may have very similar retention times. Nevertheless, retention time matches, particularly on two or more GC columns of differing polarity, can at least narrow the possibilities to a few structures, providing that standards of all possible structures are available for comparison. This is frequently not the case, particularly for laboratories that have not accumulated a library of standards. The necessity of having a full set of standards can be circumvented by careful use of retention index data, because retention indexes, within a small margin of error, are a function only of the GC column and the operating conditions. As long as these are faithfully reproduced, the retention index of an unknown compound can be replicated in any laboratory. Thus, calculation of the retention index of an unknown compound and comparison with tabulated values of standards run under the same conditions provides a piece of data of enormous value in the identification of an unknown. At the very least, comparison of retention indexes usually allows all but a few isomers to be eliminated from consideration. The most widely used system of retention indexes is that introduced by Kov??ts3, in which retention indexes are calculated relative to straight-chain alkane standards. Other sets of standards have also been used to a lesser extent, such as saturated fatty acid methyl esters, or acetate esters of alkanols4. Although tables of retention index data have probably been compiled by laboratories that work frequently with lepidopteran pheromones, to our knowledge these compilations have not been published. To remedy this deficiency, and to provide a resource for laboratories that cannot obtain extensive libraries of standards, we provide here the Kov??ts retention indexes (KI's) of most of the isomers of monounsaturated C12, C14, and C16 alcohols, aldehydes and acetates, determined on both nonpolar and polar GC stationary phases.   Experimental Preparation of standards for analysis The monoene alcohols, aldehydes and acetates were taken from our library of standards, assembled over many years from a number of sources, including The Pherobank (Research Institute for Plant Protection, Wageningen, The Netherlands) and the collection of the Plant Biotechnology Institute (National Research Council of Canada, Saskatoon, Sask., Canada). Stock solutions of alcohols and acetates in dichloromethane (~1mg cm-3) were checked for purity by thin-layer chromatography on silica gel (E. Merck, Type 5554 plates; Fisher Scientific, Pittsburgh, PA). When necessary, compounds were purified by chromatography on silica gel as follows. Solutions in CH2Cl2 (0.2 cm3, 1 mg cm-3)